1. Field of Art
The present invention relates to radiofrequency antennas.
2. Description of the Related Art
In mobile telephone applications the antenna should be small in size and durable. Since the antenna is a notably protruding part of a mobile telephone, it is highly probable that, on falling for example, it will hit the ground or another solid object. In addition, manufacture of the antenna should be as cost-effective as possible and suited to mass production. This means that the antenna should have as few components as possible, and these should withstand rough handling and be as easy as possible to manufacture.
Finnish Patent Application FI-951628 (LK-Products Oy), which corresponds to U.S. application Ser. No. 08/628,826, contains a description of a particular application of a helical antenna, which is relatively simple to manufacture and is also relatively durable. It comprises a support component and connector in one piece, a helical radiator which comprises a stem component which is soldered to the connector, and, surrounding the helix and support component, a layer of protective material by the design and coloring of which the external appearance of the antenna may be adjusted as desired. A second application of a helical antenna and the manufacture thereof are described in Finnish Patent Application FI-951670 (LK-Products Oy), which corresponds to U.S. application Ser. No. 08/630,040. The solution presented therein comprises a helical component, a connector and, situated around the helical component, an elastic protective casing, in which there may be a support component protruding inside the helix.
Replacement of the helical radiator by conductive patterns formed on a circuit board is known, for example, from Patent Publication GB-2 280 789. Such a structure is made up of a connector, a circuit board comprising conductive patterns and a protective casing.
The purpose of the helical and printed antennas described above has been to shorten the physical length of the antenna compared with a whip antenna having the same electrical length. By whip antenna is generally meant a straight conductor, which is connected by one end (its lower end) to a feed point. Frequently it is desired that a whip antenna be fitted to a mobile telephone in addition to the small helical or printed radiator, in order to increase the operating radius of the telephone. Since, however, a whip antenna is too long to be fitted rigidly to the mobile telephone, dual-purpose antennas are generally used in which the whip component may be pushed inside the mobile telephone when the telephone is in standby mode or when it is not in use. In the speech mode the whip may be pulled out, so that antenna function is improved.
In order for the antenna to also operate when the whip component is pushed inside, the telephone must comprise a second radiating element, which receives or transmits a signal regardless of the position of the whip component. In general a helical radiator is used as the second radiating element. The helix may be fitted to the top of the whip component or in such a way that the helical element is fixed to the frame of the radio telephone and the whip element is movable through the helix. In both cases, the helical radiator is in operation when the whip component is pushed inside the mobile telephone. When the whip component is pulled out, the functioning radiator is the whip or both the whip and the helix. Such an antenna structure, in which the whip and the helix function in combination when the whip component is extended, has been presented in, among others things, Finnish Patent Application FI-952742 (LK-Products Oy), which corresponds to U.S. application Ser. No. 08/654,687, which also presents as prior art certain other combination antenna solutions comprising a whip-helix combination. Finnish Patent Application FI-963097 (LK-Products Oy), which corresponds to U.S. application Ser. No. 08/907,297, presents a combination antenna in which the helical component is replaced by a printed radiator and which is chiefly intended for systems operating in the higher frequency ranges (1800, 1900 MHz), such as PCN (Personal Communication Network) or PCS (Personal Communication System).
The problem with the antennas described above, particularly in systems intended for the higher operating frequencies, is that there are growing demands on the precision of antenna manufacture. The physical length of a particular 1800 MHz antenna is approximately half that of a 900 MHz antenna which is similar in electrical characteristics. This means that the manufacturing tolerances for a helical element wound into the form of a cylindrical coil are decreasing, so that its manufacture as a mass product is not necessarily worthwhile any more. Also, the manufacturing tolerances for a circuit board element are becoming tighter and productivity is suffering.
A second problem with the antennas described above is the number of components included in the antenna, and consequently the number of work stages of antenna manufacture and the complexity of antenna assembly. For example, the helix in a helical antenna is wound to the desired form, the helix is put in place and attached to the connector, a support may possibly be inserted into the helix and a protective layer formed around the helix. The printed antenna manufacturing process is also multi-stage. The correct conductive pattern must be formed on the circuit board, the board must be cut to the correct size, the board must be put in place and attached to the connector and around all of this a protective casing must be placed. Furthermore, if a combination antenna is made, fitting the whip and for example the helix together demands its own type of precision. It must also be pointed out that, at each stage of manufacture of the product, errors may occur which impair the performance of the antenna, in which case the manufacturing yield is reduced or an unreasonable amount of time has to be spent on rectification of errors.
The aim of the present invention is to provide an antenna structure in which from the manufacturing standpoint includes only a few different components. The aim of the invention is also to present an antenna structure which during manufacture is easy to comply with the mechanical tolerances necessitated by the relatively small size. A further aim of the invention is to present an antenna structure which is mechanically durable. In addition, the aim of the invention is to present an antenna structure which is well suited to relatively large-scale series production.